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From Three to One
Stepping down to single phase is done to save money; not everyone needs three-phase current, which requires more infrastructure, wires and transformers. End users pay more for three-phase power, so where it isn't economically justifiable to sell three-phase power -- like in most residential areas -- the grid is converted from three-phase to single-phase, forgoing what would be, for most customers, an unnecessary expense.
From One to Three
Your home is probably not provided with three-phase power. If you want three-phase power, you can't just request it from the power company; you can, however, convert it yourself by using a phase converter to shift single-phase back into three-phase power. This is most often done to run electric motors -- often associated with light manufacturing, farm or industrial uses -- pulling from a standard municipal grid.
Efficiency of Three Phase Motors
Three-phase electric motors are more efficient than otherwise equivalent single-phase electric motors; that is to say: a three-phase motor creates more power per dollar spent. Whether your interest is in saving money or increasing power, any application that is electric motor-intensive calls for a three-phase motor and, more than likely, requires you to convert the electrical supply prior to running the motor.
Soft Starts
Just as the difference in efficiency between a two-cycle and four-cycle combustion engine can be complex, so to are the details of three-phase motor efficiency. One of the key efficiency details is worth explaining, however: the soft start. Consider your car's engine; how efficient would it be if you gave it full throttle right when you started the car? Fortunately, cars have multiple systems to prevent this undesirable event from happening. Many electric motors, however, do just that. Worse still, they actually draw more power when they're first turned on, as the motor hasn't built up any rotational inertia yet. To let the motor build up to speed more gradually, many motors are equipped with the ability to "soft start" in which they are started in phases. This is an energy-saver in some applications; it's a necessity in others to prevent large motors from overloading power grids.